1. Field of the Invention
The present invention relates to a color-image-forming medium which is constituted such that a color image is formed thereon with at least two colors, and relates to a pressure/heat-sensitive color-developing medium advantageously utilized in such a color-image-forming medium.
2. Description of the Related Art
As a conventional type of color image-forming medium, there is known a heat-sensitive multi-color-developing sheet, which is constituted such that more than two colors can be developed. In general, such a heat-sensitive multi-color-developing sheet comprises a sheet of paper coated with a heat-sensitive color-developing layer containing at least two kinds of leuco-pigment components and a color developer component. As is well known, a leuco-pigment per se exhibits no color. Namely, usually, the leuco-pigment exhibits milky-white or transparency, and reacts with the color developer, to thereby produce a given single-color (e.g. magenta, cyan or yellow). The leuco-pigment components, contained in the color-developing layer, feature different color-developing temperatures such that different colors can be obtained due to the respective color-developing temperatures.
For example, when the leuco-pigment components, contained in the color-developing layer, are composed of respective magenta- and cyan-developing leuco-pigments featuring low and high color-developing temperatures, respective magenta and blue, can be obtained due to the low and high color-developing temperatures thereof. In particular, when a first temperature between the low magenta-developing temperature and the high cyan-developing temperature is locally exerted on the color-developing layer, only the magenta-developing leuco-pigment component reacts with the color developer component so that magenta is developed at the localized area where the first temperature is exerted on. Also, when a second temperature higher than the high cyan-developing temperature is locally exerted on the color-developing layer, both the magenta- and cyan-developing leuco-pigment components react with the color developer component so that blue is developed as a mixture of magenta and cyan at the localized area where the second temperature is exerted on.
As is apparent from the aforesaid example, it is impossible to independently develop cyan by the cyan-developing leuco-pigment component. Thus, the conventional multi-color image-forming medium is inferior in efficiency of color development, because it is possible to independently develop only a leuco-pigment component exhibiting the lowest color-developing temperature.
Also, in the aforesaid example, a temperature difference between the low magenta-developing temperature and the high cyan-developing temperature must be sufficiently high, before a development of pure magenta can be obtained on the color-developing layer. Namely, if the temperature difference between the magenta-developing temperature and the cyan-developing temperatures is too low, a part of the cyan-developing leuco-pigment component may undesirably react with the color developer component at the first temperature for the development of magenta, resulting in the development of magenta with a cyan tint.
Further, in the aforesaid example, the low magenta-developing temperature must be more than 100° C., before erroneous and accidental development of magenta can be prevented, because the color-developing layer may be frequently exposed to, for example, a temperature in a range of 80 to 100° C. under an ordinary circumstance. Thus, if the low magenta-developing temperature is less than 100° C., the erroneous and accidental development of magenta may often occur.
Accordingly, in the conventional multi-color image-forming medium, a combination of different leuco-pigments, which can be utilized to form a heat-sensitive color-developing layer, is severely and considerably restricted, because respective various leuco-pigments feature inherent color-developing temperatures. In the aforesaid example, if one is optionally selected from among various magenta-developing leuco-pigments, it cannot be ensured whether there is a cyan-developing leuco-pigment which can be combined with the selected magenta-developing leuco-pigment.
Conventionally, although a user frequently requires that only one single-color is developed with a desired tone in a multi-color image-forming medium, it is virtually impossible to even obtain the development of only the single-color with the desired tone, because of the severe and considerable restriction of the combination of different leuco-pigments.
Further, the conventional color image-forming medium is inferior in thermal energy efficiency for the development of color, because the lowest color-developing temperature must be more than 100° C., before erroneous and accidental development of color can be prevented, and because the temperature difference between the low color-developing temperature and the high color-developing temperature must be sufficiently large.
Furthermore, in the conventional multi-color image-forming medium, of course, it is impossible to utilize a pigment type other than a leuco-pigment.